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People like Elon Musk and others in the "space settlement" camp believe NASA and the US space industry should colonize Mars and make homo sapiens a multi-planet species. Musk wants SpaceX to one day send colonists to Mars, while NASA is talking about building sustainable habitats on the red planet. Even if we overcome the technical and financial challenges that stand in the way of putting humans on Mars, we know precious little about how microgravity and heightened radiation will affect the ability of humans to reproduce in space. Without procreation, there is no permanent colony, and early tests aren’t promising.

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But let’s assume SpaceX, or an international coalition led by NASA, or China’s space agency eventually figure out the engineering and financing of a Mars colony. Let’s also assume the biology of reproduction in space, and on foreign worlds, is a solvable problem. After homo sapiens becomes a multi-planet species, the question becomes, would we remain a single species of humanity? Scott Solomon thinks a lot about this question in his new book Future Humans, which will be published by Yale Press in October. In it, he explores the future evolution of our species, including some musings on Mars.

“The general concept for the book is to ask about our ongoing evolution, from the perspective of a scientist who takes what we know about our past, what we know about today, and thinking about the long-term possibilities for our species,” Solomon, a biologist at Rice University in Houston, said. What, he wondered, would it take to lead to development of a new species? Put another way, how long would humans on Mars remain human?

Martian speciation

Solomon explained that new species evolve most commonly when a barrier prevents a population from mating, such as on an island archipelago, so species on separate Galapagos islands evolve along separate lines. With modern humanity, of course, the trend is going in the opposite direction, as people move around the planet at a rate unprecedented in human history. “So on planet Earth it would take a major change to imagine us having populations isolated long enough to have distinct species,” he said.

The gulf between Earth and Mars might present such a barrier, if the Martian colony were self-sustaining and persistent. Through natural selection, humans and any organisms they bring with them, such as a plants, may evolve and adapt to Mars' harsh environment and low gravity, which is only a third of Earth's gravity.

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Lacking a magnetosphere, Mars is bombarded by an increased rate of radiation, which also favors speciation. Ionizing radiation causes mutation in genes, which would provide a source of new genetic variations. That could accelerate the process of adaptation. On the downside, Solomon said, the higher radiation might just kill people. Or it might cause colonists to perpetually huddle inside small habitats and space suits, leading a Morlock-like existence and facing a similar evolutionary fate.

Ultimately it still may take a long time for speciation to occur. The one solid data point we have on Earth is the colonization of the Americas, which were settled by waves of people moving across the Bering Strait around the end of the last ice age. These populations were then isolated from the rest of world for about 10,000 years. When Europeans arrived they found a distinct population of native Americans, Solomon said, but certainly not a different species. That would suggest that, on a planet with a similar atmosphere and gravity as the Earth, it would take a human population more than 10,000 years to speciate. Mars is not that planet, of course.

Another factor to consider as humans contemplate colonizing other worlds, Solomon said, is the “founder effect,” which simply means that when a small number of people establish a new population from a larger population, the genes of the founders will have a huge influence on that population moving forward. This occurred with the small bands of humans spreading out from Africa.

“I’m thinking about what the long-term fate of our species may be,” Solomon said. “When selecting colonists I don’t believe we should be trying to select what attributes we want in a new species of humans. But it’s interesting to think that if you were to take only people from certain populations, or try to include a diversity of all of humanity, how those outcomes would be very different for the potential of what might become a new species of humans.”